An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, an atrial tachycardia is typically not fatal. However, atrial tachycardia reduces blood flow from the heart, increasing fatigue. Additionally, over the long term, atrial tachycardia can lead to other problems including stroke.
One technique for preventing atrial arrhythmias is dynamic atrial overdrive (DAO) pacing wherein an implantable cardiac stimulation device, such as a pacemaker or implantable cardioverter/defibrillator (ICD), applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic atrial heart rate of the patient. In one example described in U.S. patent application Ser. No. 09/471,788, filed Dec. 23, 1999, and entitled “Methods And Apparatus For Overdrive Pacing Heart Tissue Using An Implantable Cardiac Stimulation Device”, the stimulation device monitors the heart of the patient and, if two consecutive intrinsic heart beats are detected, overdrive pacing is automatically triggered. The overdrive pacing rate is based on the heart rate detected at the time overdrive pacing is triggered and is typically 5 to 10 pulses per minute (ppm) higher than the intrinsic rate. The intrinsic heart rate may be determined, for example, by calculating the time interval between the two consecutive intrinsic beats. The stimulation device then overdrive paces the heart at the selected overdrive pacing rate for a programmed number of overdrive events or “overdrive cycles”. Thereafter, the stimulation device slowly decreases the overdrive pacing rate by a rate decrement specified by a programmed “recovery rate” until additional intrinsic beats are detected, then the device repeats the process to determine a new overdrive pacing rate and pace accordingly. In this manner, the heart is paced, at most times, at a rate slightly faster than the intrinsic rate and so relatively few intrinsic heart beats occur.
It is believed that overdrive pacing is effective for at least some patients for preventing or suppressing the onset of tachyarrhythmias for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods which, in turn, can trigger a tachyarrhythmia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered more uniform and periodic. Thus, the dispersion of refractory periods is reduced and episodes of tachyarrhythmia are generally avoided.
Thus it is desirable within patients prone to tachyarrhythmia to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive pacing rate. However, a high overdrive pacing rate has disadvantages as well. For example, a high overdrive pacing rate may be unpleasant to the patient, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high heart rate may also cause possible damage to the heart or may possibly trigger a more serious arrhythmia, such as a ventricular fibrillation. A high overdrive rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. A high overdrive rate may actually exacerbate heart failure in these patients. Also, a high overdrive rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete a power supply of the implantable cardiac stimulation device, perhaps requiring frequent surgical replacement of the device.
Thus it is critical that the various overdrive pacing parameters (the overdrive rate, the number of overdrive cycles, the recovery rate) be set so as to achieve an optimal level of overdrive pacing sufficient to reduce the risk of a tachyarrhythmia without undue side effects. These overdrive pacing control parameters are programmed by a physician or other clinician using an external programmer device, which electronically transmits programming commands to the implanted stimulation device. Typically, the physician seeks to program the overdrive control parameters to values that will achieve a percentage of paced vs. sensed beats of 85% to 95%, i.e. 85% to 95% of the total heart beats are paced beats whereas only 5% to 15% are intrinsic beats. Unfortunately, it is quite difficult for a physician to initially determine the parameters needed to achieve a desired degree of overdrive pacing within a particular patient. Instead, the physician typically sets the various control parameters of the stimulation device of the patient to default values and then programs the device to record the resulting degree of overdrive pacing as a function of heart rate. The patient is sent home and, weeks or months later, the patient returns to the physician for a follow-up session to permit the physician to review the recorded information and to determine whether the default parameters achieved the desired degree of overdrive pacing. If the percentage of paced vs. sensed beats is too low, perhaps only 50%, the physician then either decreases the recovery rate or increases the overdrive rate or number of overdrive cycles or makes a combination of adjustments. If the percentage of paced vs. sensed beats is too high, perhaps 100%, the physician either increases the recovery rate or decreases the overdrive rate or number of overdrive cycles. The patient is again sent home and, weeks or months later, the patient again returns to the physician so that the physician can again review the recorded percentage of paced vs. sensed beats and, if needed, adjust the overdrive control parameters again. This process is usually repeated several times over a period of many months until a set of overdrive pacing control parameters are identified that come closest to achieving the desired percentage of paced vs. sensed beats.
As can be appreciated, it would be desirable to provide more effective techniques for permitting a physician to more easily determine the optimal set of control parameters so that repeated office visits are not required and so that the patient more promptly receives optimal overdrive pacing. One particular area of concerns relates to the diagnostic information provided to the physician via the external programmer that pertains to overdrive pacing. Typically, no diagnostic information specifically pertinent to overdrive pacing is detected by the implanted device and subsequently made available to the physician via the external programmer. Rather, only conventional types of diagnostic information, such as the percentage of paced vs. sensed beats, is collected. Hence, the physician must make judgments regarding the effectiveness of the overdrive pacing parameters based on limited diagnostic information. As one example, although optimal setting of the recovery rate is often critical for achieving effective overdrive pacing, conventional implantable devices typically do not provide diagnostic information directly relevant to setting the recovery rate, such as the longest recorded recovery duration or the maximum rate associated with the longest recovery duration.
Accordingly, it would be highly desirable to provide an improved implantable cardiac stimulation device capable of tracking and recording diagnostic information specifically pertinent to overdrive pacing and to provide an improved external programmer capable of processing and displaying the overdrive pacing diagnostic information. It is to these ends that the invention is primarily directed.